With industrial development, a huge amount of plastic products are used worldwide. Last year, Korea produced about 7 million tons of general-use plastic products and became the world's 4th largest plastic producing nation. However, waste plastics are causing many environmental problems. Currently, waste plastics are mostly landfilled in the ground.
Deposition of waste plastic causes serious environmental problems such as long biodegradation time in soil, lack of landfill site, or the like. Thus, recycling of waste plastics as resources is drawing a lot of attentions. Among many solutions having been proposed for disposal of waste plastics, recycling of them as fuel oil or available feedstocks such as market chemicals is considered as the most preferable solution in aspect of economic and environment, especially compared with simple mechanical recycling followed by physical additions or processing.
Recycling of waste plastics may comprises material recycling (recycling as product material), thermal recycling (recycling into electricity, heat or other energy), and chemical recycling (recycling in the form of chemicals material such as resin material).
Physical recycling is mainly utilized in production of recycled resin, light-weight concrete, adhesive, etc. But, the newly added value by the physical recycling is not significant and recycling may be impossible after repetition of several physical recycling processes. Further, it is disadvantageous in ultimately giving a large quantity of waste polystyrene. And, the contaminated waste polystyrene foams from the wholesale markets of agricultural and fishery products or the construction sites are inappropriate for the physical recycling because they are not clean as compared to other waste polystyrene.
Moreover, because the contaminated waste polystyrene foams are about 50 times larger in volume than other waste polystyrene, those are not suitable for physical recycling, and thus have been landfilled in the ground or incinerated. However, the incineration is not environmentally friendly since dioxin or other toxic substances may be produced during the incineration.
Therefore, chemical recycling is drawing attentions. Recovery of styrene monomer from waste polystyrene was first attempted in 1997 by Nishizaki et al. It was reported that about 50% of styrene could be recovered from waste polystyrene by pyrolysis at 733 K. Since then, a lot of studies have been made to improve the yield of styrene.
FIG. 6 schematically shows a conventional process for recovery of styrene monomer from waste polystyrene by pyrolysis.
As illustrated in the figure, an existing apparatus for recovery of styrene monomer from waste polystyrene includes an injection molder, a melter, a reactor, a heat exchanger and an oil reservoir. Waste polystyrene pulverized passing through the injection molder is melted as it passes through the melter. After pyrolysis in the reactor, the resultant gas is condensed as it passed through the heat exchanger and is collected in the oil reservoir. The existing apparatus for recovery of styrene monomer is disadvantageous in that the recovery ratio of styrene monomer decreases as the pyrolysis proceeds because of residues produced from the reaction.
This will be described in more detail. According to the existing method, pyrolysis is performed continuously using a continuous stirred-tank reactor (CSTR) illustrated in FIG. 7 in order to pyrolyze the waste polystyrene melted by the melter.
As the waste polystyrene is continuously pyrolyzed in the CSTR reactor for a long time, residues are formed accumulatively and, as a result, the production of styrene monomer is interfered and the decomposed styrene monomer may react with the residue to form unwanted byproducts such as ethylbenzene, α-methylstyrene (AMS), benzene and toluene.
Also, in the existing method, the gas produced from the pyrolysis reactor is condensed by indirect cooling using a coolant. However, the indirect cooling is not so effective and styrene monomer may be repolymerized, thereby resulting in decreased yield of styrene monomer.